Groundwater Development and Geothermy

con-slot is the TRADE MARK for well screens and industrial process screens. All con-slot products are being manufactured in our modern plant, located in the industrial area of the City of Wittingen on the banks of the Elbe-Side-Canal. Thus, quick connections to the seaport of Hamburg and all overseas markets are guaranteed. Further, we offer airfreight service worldwide via the airports of Hanover and Hamburg.

Precise manufacturing, excellent workmanship and quick delivery are the columns of our reputation. Our manufacturing programme comprises all components needed to complete water wells:

Our standard programme is nominal size 1" (25 mm) to 34" (850 mm); however we do also manufacture larger diameters upon re­quest. con-slot design meets always the corre­sponding requirements.

Our experience is based on technical ad­vice, structural design and installation supervision throughout many countries of the world, thus guaranteeing the most econ­omical solution for water wells. In our labora­tory our technical staff has the possibility to determine the right material, optimum strength and correct slot sizes of the con-slot high efficiency well screen.

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Screen dimensions for DIN 4925/2458

delivery conditions:





NS 50 - NS 80 in length up to 5.00 m
NS 100 - NS 850 in length up to 6.00 m
over NS 850 upon request


standard structure
heavy structure


recommended max. installation depth up to 100 meter
recommended max. installation depth up to 300 meter
beyond 300 meter installation depth with heavy duty structure to be agreed upon




1 = surface-(wrapping)profiles
2 = support-profiles
3 = numbers of supports on the circumference




weight kg/m
weight kg/set



net-weight of screens calculated with slot width of 0.300 mm
net-weight of threaded connectors according to API 5CT/ 5B BUTTRESS
net-weight of flanges according to DIN 4922

    shipment weight, including packing, use multiplier of 1,15 for approximation

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Screen dimensions for DIN 4925

Standard structure*

con -slot high efficiency water well screens of profile wire wrapped all-welded structure, inside diameter with reference to DIN 4925 to allow the installation combination with PVC-sump and extension pipes :

- slot sizes equal or smaller 0.500 mm allow the structural use of the wrapping profile (1) one
size smaller, thus increasing the entrance area!
- I.D.FI = inner diameter of well screen
- I.D.PVC = inner diameter of plastic PVC pipe

Grafik 1

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Screen dimensions for DIN 2458

Heavy structure*

con -slot high efficiency water well screens of profile wire wrapped all-welded structure, dimensions corresponding to the ISO Standard for pipes, i.e. DIN 2458/DIN 2463, to allow the combination with steel and stainless steel sump pipes and extension pipes:

Grafik 2

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Tables K-Factor of con-slot high efficiency water well screens of 'Standard Structure'

Free entrance area  f x 100 = %
Permeability factor k1=cm/sec

DN = nominal diameter
O.R. = outer radius of screen
I.R. = inner radius of screen
w = effective wall-thickness of screen
sw = slot width

Grafik 3

Grafik 4

Grafik 5

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Tables K-Factor of con-slot high efficiency water well screens of 'Heavy Structure'

Free entrance area  f x 100 = %
Permeability factor k1=cm/sec

DN = nominal diameter
O.R. = outer radius of screen
I.R. = inner radius of screen
w = effective wall-thickness of screen
sw = slot width

Grfaik 6#

Grafik 7

Grafik 8

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How to find proper screen dimensions and correct slot sizes

Specific capacity and available drawdown indicate the productivity of a well to be expected.
If the pump is installed above the screen the rising velocity at the screen head should not exceed v = 1.75 meter/sec.
If the pump is installed below the screen in a pump sump then the screen diameter depends mainly on the pump size and the remaining annulus space between the screen and the riser pipe.

Screen length
Once decided on the screen diameter the laminar flow conditions at the entrance of the screen determine the requested length. Screen length in the total height of a water bearing formation is - especially when using the con-slot screen - not necessary. The Kozeny concept of investigation for partial penetrated formation is an excellent practice to determine max. necessary screen length.

Grafik 9

Curves of Kozeny formula
Curves for calculating the specific
capacity of partially penetrated aquifers

Slot width

First of all it has to be decided whether a natural development or a desanding through a gravel pack should be performed. Several different criterias are necessary to make the right decision:

1) In case of choosing natural development the uniformity coefficient d 60: d 10 of the formation sand sieve analysis determines selection of the slot width.

Grafik 10

Natural development without gravel pack
Slot design with natural desanding

In this case is critical:
If U = 2.5 to 4.0, the slot width is found at the intersection of 40% cumulative retained with the sieve analysis curve.
If U =/smaller 2.5, the slot width is found at the intersection of 50% cumulative retained with the sieve analysis curve.
If U =/above 4.0, the slot width is found at the intersection of 30% cumulative retained with the sieve analysis curve.

The larger the open area of a screen, the longer lasts the time of development (desanding). The better the productivity of the well!

Another important information for well screen installations with natural develop­ment:
formations, where fine sand formations are on top of coarse sand formations, two thumb rules should be considered:

a) the screen should extend approx. 0.50 m with the smaller slot into the coarser formation below.
b) the slot size designed for the coarser formation should not be more than 2 times the slot size for the overlying finer formation.

2) In case of choosing a gravel pack, the granulometric structure of the gravel must be in relation to the formation material. Also in this case different criterias of selection are imperative:
the gravel pack thickness should not exceed 15 cm (6"): each further cm reduces the effectiveness of the desanding operation. The most favourite thickness is 5.0 to 7.5 cm.
The coefficient of uniformity U of the gravel pack should be possibly smaller/= 2.0.

Grafik 11

Natural development with gravel pack
Method for gravel pack design and corresponding slot size

First step is to determine the critical grain size of formation material by a sieve analysis:
Critical grain size is at 70 % cumulative retained if uniformity coefficient is less /= 4.0 and the entrance velocity in the slot is smaller /= 0,03 m/sec;
Critical grain size is at 40 % cumulative retained if uniformity coefficient is more than 4.0 and the entrance velocity in the slot is above /= 0.03 m/sec.

In order to determine the critical grain size of a gravel pack, a multiplier of 6 to 8 has to be applied on the critical grain size of the formation material.

A multiplier under 6 does hardly allow the desanding of the formation, reduces the permeability around the screen and as such the efficiency of the well.

A multiplier above 8 can cause part penetration of formation material into the gravel pack thus reducing its porosity and as such also the efficiency of the well.

Experience has proven a multiplier of 6 to be the optimum in most applications. A multiplier of 8 should be considered as max. upper limit and should only be chosen if the coefficient of uniformity of the formation is above 4.0, i.e. when for­mation is coarse and nonuniform.

Determination of the gravel pack is reached by designing a curve with a uniformity coefficient of approx. 2.0 (d40/d90) through the critical grain size. This curve should be adapted to an analysis of a commercially available gravel pack.
The intersection of the gravel pack curve with the 90% cumulative retained finally determines the slot width for the screen. This is optimum design of slot width for well screens.

However, experienced professionals choose often, for greater safety, the slot width determined for natural development as explained under item 1) above. This is due to the fact that formation material samples quite often are not representative.
Even with a smaller slot, a con-slot high efficiency water well screen is more permeable than the surrounding gravel pack and/or water bearing stratum.

Just this is the advantage of this screen type!

But always care should be taken to really choose an uniform gravel pack in order to bring the desanding of the formation to a success and to ensure that the gravel pack will not loose its porosity by part penetration of formation material!

The above guidelines for the slot design will be optimum if the formation samples for the analysis are representative, corresponding to the natural structure of the water bearing formation - as undisturbed as possible.

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Well Design - Sample Calculation

con-slot has a sample calculation available upon request, elaborated for a typical geological formation in Northern Europe.

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Installation and Development Methods (Intensive Desanding):

If the slot width has been selected considering the recommended criterias on part “How to find proper screen dimensions and correct slot sizes” and in correct relation with the formation ma­terial, then the intensive desand­ing will ensure always balanced hydraulic conditions at the well's perimeter and at the entrance flow into the well screen.

No other screen type offers so much the possibility of a homogeneous structural im­provement of the formation around the well screen and thus an increase of storage ca­pacity like the con-slot high efficiency well screen with its continuous slot. Each slot is only separated from the next by a small profile allowing to rearrange the gravel pack and/or formation on the entire perimeter practically millimetre by millimetre.

Natural development of a well means de-sanding and simultaneously stabilization of the surrounding water bearing formation. This in combination with a con-slot high ef­ficiency well screen means sand free water pumping with laminar flow at the entrance to the screen for the whole operational time of a well.

Here it must also be mentioned that in case it has been decided to use natural develop­ment because of uniform formation con-slot screens can be vibrated or jetted into the ground if installation depth is shallow, an easy and simple method.

For natural development or intensive desanding, there are three methods available with different results:

1) Surge Plunger
Surge plunger, a piston with several rub­ber seals in order to seal against the inner wall of the well screen, is rising and falling in conjunction with the stroke of the percussion of the drilling rig. When dropping the water is pressed through the screen into the surrounding formation thus building a surpressure. When lifting the fines are sucked through the slot into the well and deposited in the sump. From there the fines must be pumped from the well in a second tour.

Grafik 12

Surge Plunger

2) Compressed Air
Compressed air is produced by a compressor and is pressed into the formation dosed in proportion by an air vessel. In addition this procedure is intensified by sealing part length of the screen with rubber seals. The air penetrates the formation more or less depending on the porosity and degree of uniformity, and thus producing a surcharge of pressure.

By a sudden pressure relief the air cushion built-up in the formation shoots back into the screen loaded with the fines from the formation.

The surcharge of pressure and the high air speed delivers the fines simultaneously from the well to the surface directly into the control bin. An additional tour to clean the well from fines is not necessary.

The capacity of the compressor should be min. 7 bar. However, the more capacity the compressor has, the lesser time is needed for desanding.

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Tube Dimensions for Compressed Air Intensive Desanding

Grafik 13

Grafik 14

Compressed air intensive desanding (system: Celler Brunnenbau GmbH)

3) Power Stream Jetting
Power stream jetting can only be applied at screens with continuous slots, where the energy of the water jet is not reduced or deviated by large blind areas of screen walls.

The continuous V-shaped slot allows the water jetting to thrust directly into the gravel pack and/or formation without any loss of energy or pressure. The jetted water penetrates with full power the surrounding formations. Simultaneous pumping withdraws the fines of the formation into the well and conveys it directly to the surface into the control bin. As the picture shows, a submersible pump can be used for this method. One should consider that it is not necessary to work with the expected well efficiency. The capacity of the submersible pump can be less. Yet the pump should allow to work with high pressure. Above the submersible pump a jetting tool with 4 nozzles is installed in the riser pipe, thus allowing with a 90 degree turn to reach the entire perimeter of the well screen surface. The 90 degree turn can be reached by means of a swivel installed in the riser pipe above the surface. A gate or flap valve and a pressure gauge at the discharge end above the control bin allow the precise dosing of the jetting/pumping procedure.

By closing the valve the pressure on the jetting tool is increased thus allowing deep penetration into the gravel pack and/ or formation.

By opening the valve the pressure is relieved from the formation and the fines trans­ported through the slots into the well screen. On the other hand the pumping capacity is increased simultaneously and the fines are pumped from the well into the control bin.

This method allows to find the most proper conditions for the well development by searching the ideal parameter on the flow/pressure curve of the pump. The pressure gauge allows continuous control of the procedure.

Experience proves this method to be the most effective one of all desanding methods. It does not only give high performance but also is the fastest.

Abrasion of rotors and rotor blocks of the pump are negligible so that this method can be considered as the most economic of the three methods described.

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Jet head discharge and pressure for High Stream Desanding per jet head and nozzle size

Grafik 15

Grafik 16

Power stream jetting

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con-slot SCREENS | Industriegebiet Hafen | Graue Riethe 2 | D-29378 Wittingen
Tel: +49 5831 2515-0 | Mail: